Philips Semiconductors Product specification
P83C557E4/P80C557E4/P89C557E4Single-chip 8-bit microcontroller
1999 Mar 02
32
The Control Register, S1CON:
The CPU can read from and write to this 8-bit, directly addressable
SFR. Two bits are affected by the SIO1 hardware: the SI bit is set
when a serial interrupt is requested, and the STO bit is cleared when
a STOP condition is present on the I
2
C bus. The STO bit is also
cleared when ENS1 = 0.
Figure 30. Serial control (S1CON) register.
76543210
S1CON (D8H) CR2 ENS1 STA STO SI AA CR1 CR0
Table 24. Description of S1CON bits
SYMBOL BIT FUNCTION
CR2 S1CON.7 Clock rate bit 2, see Table 25.
ENS1 S1CON.6 ENS1 = 0: Serial I/O disabled and reset. SDA and SCL outputs are high-Z.
ENS1 = 1: Serial I/O enabled.
STA S1CON.5 START flag. When this bit is set in slave mode, the hardware checks the I
2
C bus and generates a START
condition if the bus is free or after the bus becomes free. If the device operates in master mode it will
generate a repeated START condition.
STO S1CON.4 STOP flag. If this bit is set in a master mode a STOP condition is generated. A STOP condition detected on
the I
2
C bus clears this bit. This bit may also be set in slave mode in order to recover from an error
condition. In this case no STOP condition is generated to the I
2
C bus, but the hardware releases the SDA
and SCL lines and switches to the not selected receiver mode. The STOP flag is cleared by the hardware.
SI S1CON.3 Serial Interrupt flag. This flag is set, and an interrupt request is generated, after any of the following events
occur:
– A START condition is generated in master mode.
– The own slave address has been received during AA = 1.
– The general call address has been received while S1ADR.0 and AA = 1.
– A data byte has been received or transmitted in master mode (even if arbitration is lost).
– A data byte has been received or transmitted as selected slave.
– A STOP or START condition is received as selected slave receiver or transmitter.
While the SI flag is set, SCL remains LOW and the serial transfer is suspended. SI must be reset by software.
AA S1CON.2 Assert Acknowledge flag. When this bit is set, an acknowledge is returned after any one of the following
conditions:
– Own slave address is received.
– General call address is received (S1ADR.0 = 1).
– A data byte is received, while the device is programmed to be a master receiver.
– A data byte is received. while the device is a selected slave receiver.
When the bit is reset, no acknowledge is returned. Consequently, no interrupt is requested when the own
address or general call address is received.
CR1
CR0
S1CON.1
S1CON.0
Clock rate bits 1 and 0, see Table 25.
